This Core is designed to directly support the research objectives of Projects 1,2,3, and 4. Two murine models of persistent infection will be employed. Adult nude mice will be used because of their propensity to develop extra-intestinal crypotsporidial infections, particularly within the biliary duct system. Adult scid mice will also be used because of the ease with which passive antibody transfer and lymphoid cell transplantation protocols can be performed. The specific aims of this Core Component are as follows: 1. Utilize persistently infected nude mice for treatment of extra- intestinal cryptosporidiosis with monoclonal IgA neutralizing antibodies obtained from Project 4. 2. Utilize persistently infected scid mice for evaluation of protective immune responses induced by antigens obtained from Project 3. 3. Test defined antibodies from Projects 1 and 2 for ability to terminate persistent cryptosporidial infection in scid mice.